[Expression Language] fix foo[index]

| Q | A |
|----|-----|
| Bug fix? | yes |
| New feature? | no |
| BC breaks? | no |
| Deprecations? | no |
| Tests pass? | yes |
| Fixed tickets | |
| License | MIT |
| Doc PR | |

This patch fixes the usage of variables as key in GetAttrNode.

```php
$language = new \Symfony\Component\ExpressionLanguage\ExpressionLanguage();
$language->evaluate('foo[i]', ['foo' => ['a', 'b', 'c'], 'i' => 1]); // 'b'
```